EXPERIMENTAL TEST OF ENVIRONMENTAL-FACTORS INFLUENCING THE SEAWARD MIGRATION OF EUROPEAN SILVER EELS

In this study we test which environmental factors are influencing the migratory behaviour of seaward migrating silver European eels Anguilla anguilla in a Norwegian river. We test this by transplanting tagged s ilver eels upriver after catching them in a trap at the outlet of the river. We did this at four fixed dates (fixed day lengths) during 5 ye ars. Over 74% of the variation in time from release to subsequent reca pture could be accounted for by variation in time at release, water di scharge at release, and moon phase at release (in decreasing order of importance). The number of days to median day of recapture of each bat ch decreased as the season progressed and decreased with increasing wa ter discharge. Water temperature did not explain any of the remaining variation in the model, even if there was a significant correlation be tween water temperature at release and median day at recapture of each batch (in simple linear regression model). Recapture rate during the same migration season varied strongly with water temperature, but at m ost 33% of the Variation could be explained by this factor. It was als o evident that the recapture rate increased when the number of downwar d migrating untagged silver eels was high. Day length (i.e. time at re lease) did not explain any of the remaining variation in recapture rat e.